Laminated metals, and in particular nanolaminated metals, are of interest for structural and thermal applications because of their unique toughness, fatigue resistance and thermal stability. For corrosion protection, however, relatively little success has been reported in the formation of corrosion-resistant coatings that are laminated on the nanoscale.
Electrodeposition has been successfully used to deposit nanolaminated coatings on metal and alloy components for a variety of engineering applications. Electrodeposition is recognized as a low-cost method for forming a dense coating on any conductive substrate. Electrodeposition has been demonstrated as a viable means for producing nanolaminated coatings, in which the individual laminates may vary in the composition of the metal, ceramic or organic-metal composition or other microstructure feature. By time varying electrodeposition parameters such as current density, bath composition, pH, mixing rate, and/or temperature, multi-laminate materials can be produced in a single bath. Alternately by moving a mandrel or substrate from one bath to another, each of which represents a different combination of parameters that are held constant, multi-laminate materials or coatings can be realized.
The corrosion behavior of organic, ceramic, metal and metal-containing coatings depends primarily on their chemistry, microstructure, adhesion, thickness and galvanic interaction with the substrate to which they are applied. In the case of sacrificial metal or metal-containing coatings, such as zinc on an iron-based substrate, the coating is less electronegative than the substrate and so oxidation of the coating occurs preferentially, thus protecting the substrate. Because these coatings protect by providing an oxidation-preferred sacrificial layer, they will continue to work even when marred or scratched. The performance of sacrificial coatings depends heavily on the rate of oxidation of the coating layer and the thickness of the sacrificial layer. Corrosion protection of the substrate only lasts so long as the sacrificial coating is in place and may vary depending on the environment that the coating is subjected to and the resulting rate of coating oxidation.
Alternately, in the case of a barrier coating, such as nickel on an iron-based substrate, the coating is more electronegative than the substrate and thus works by creating a barrier to oxidative corrosion. In A-type metals, such as Fe, Ni, Cr and Zn, it is generally true that the higher the electronegativity, the greater the nobility (non reactivity). When the coating is more noble than the substrate, if that coating is marred or scratched in any way, or if coverage is not complete, these coatings will not work, and may accelerate the progress of substrate corrosion at the substrate: coating interface, resulting in preferential attack of the substrate. This is also true when ceramic coatings are used. For example, it has been reported in the prior art that while fully dense TiN coatings are more noble than steel and aluminum in resistance to various corrosive environments, pinholes and micropores that can occur during processing of these coating are detrimental to their corrosion resistance properties. In the case of barrier coatings, pinholes in the coating may accelerate corrosion in the underlying metal by pitting, crevice or galvanic corrosion mechanisms.
Many approaches have been utilized to improve the corrosion resistance of barrier coatings, such as reducing pinhole defects through the use of a metallic intermediate layer or multiple layering schemes. Such approaches are generally targeted at reducing the probability of defects or reducing the susceptibility to failure in the case of a defect, mar or scratch. One example of a multiple layering scheme is the practice commonly found in the deployment of industrial coatings, which involves the use of a primer, containing a sacrificial metal such as zinc, coupled with a highly-crosslinked, low surface energy topcoat (such as a fluorinated or polyurethane topcoat). In such case, the topcoat acts as a barrier to corrosion. In case the integrity of the topcoat is compromised for any reason, the metal contained in the primer acts as a sacrificial media, thus sacrificially protecting the substrate from corrosion.
Dezincification is a term is used to mean the corroding away of one constituent of any alloy leaving the others more or less in situ. This phenomenon is perhaps most common in brasses containing high percentages of zinc, but the same or parallel phenomena are familiar in the corrosion of aluminum bronzes and other alloys of metals of widely different chemical affinities. Dezincification usually becomes evident as an area with well-defined boundaries, and within which the more noble metal becomes concentrated as compared with the original alloy. In the case of brass the zinc is often almost completely removed and copper is present almost in a pure state, but in a very weak mechanical condition. Corrosion by dezincification usually depends on the galvanic differential between the dissimilar metals and the environmental conditions contributing to corrosion. Dezincification of alloys results in overall loss of the structural integrity of the alloy and is considered one of the most aggressive forms of corrosion.
Coatings that may represent the best of both the sacrificial coating and the barrier coating are those that are more noble than the substrate and creates a barrier to corrosion, but, in case that coating is compromised, is also less noble than the substrate and will sacrificially corrode, thus protecting the substrate from direct attack.